Giving overview of human embryonic development including spermatogenesis, oogenesis, fertilization, gastrulation, cleavage, extraembryonic layers and pregnancy

1. GAMETOGENESIS, FERTILIZATION AND EMBRYONIC
DEVELOPMENT IN HUMAN
NEELAM DEVPURA
(M.Sc., NET, GATE)
Ph.D. Scholar 1

2. CONTENT
Gametogenesis
a. spermatogenesis
b. oogenesis
Hormonal control of spermatogenesis and
oogenesis
Fertilization
Cleavage
Gastrulation
Embryo development
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3. GAMETOGENESIS
 Both mitosis and meiosis play a role in gametogenesis.
Mitosis provides the precursor cells. Meiosis brings about
the reduction divisions that result in gametes.
 Special cells, primordial germ cells, in the gonads divide,
grow, divide again and then differentiate into the gametes.
 Sperm produced in testes, Ova produced in the ovaries.
 In human males gametogenesis continues constantly from
puberty. In females the mitotic divisions take place before
birth. The meiotic divisions take place in a few oocytes each
monthly cycle from puberty to menopause.
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4. Reproductive anatomy of human male & female
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5. MALE GAMETOGENESIS
SPERMATOGENESIS SPERMIOGENESIS
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6. • 1st stage: formation of
spermatocytes
– Spermatogonia are stem cells
• Least differentiated (earliest
in the process)
• Lie in basal lamina
– Divide continuously by mitosis
(result 2n or diploid): daughter
cells A (remains a stem cell) or
B (goes on)
– When start to undergo meiosis
are by definition called
spermatocytes
• 2nd stage: meiosis I
– Each primary spermatocytes
(2n) undergoes meiosis I to
become 2 secondary
spermatocytes:
– Each secondary spermatocyte
undergoes meiosis II to become
2 spermatids
– Therefore 4 total spermatids
from each spermatogonium
• 3rd stage: spermiogenesis
– Spermatids differentiate into
sperm
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7.  Sperm can swim only after they have left the testis
 Process of spermatogenesis is controlled by two hormones
 FSH (follicle stimulating hormones) from anterior pituitary
 Testosterone
 primary male hormone
 produces by testes
 Sperm surrounded by Sertoli (sustenacular) cells: tight junctions
• “Nurse” cells of the testes.
• Activated by FSH.
• Located in convoluted seminiferous tubules.
• Main function is to nourish the developing sperm cells.
 Interstitial or Leydig cells: secrete androgens
 Male sex hormones
 Main one is testosterone
 Into blood, sustain all male sex characteristic and sex organs
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9. Oogenesis
Generation of eggs
• Starts in fetal period
– No more oocytes made after about 7th month
– Developed only to early stage of meiosis I by birth and stops (called
primary oocyte)
• 6-12 primordial oocytes each cycle selected to develop for ovulation (most die)
– Only then is meiosis I completed
– Secondary oocyte is then arrested in meiosis II
• Meiosis II not completed (now an ovum) unless sperm penetrates its plasma
membrane
• Of the 4 daughter cells, only one becomes ovum (needs a lot of cytoplasm)
– The other 3 become “polar bodies”
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10. Hormonal Control of Spermatogenesis
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11. Hormonal Control of Oogenesis
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12. Spermatogenesis differs from oogenesis in three
ways
– All four products of meiosis develop into sperm
while only one of the four becomes an egg.
– Spermatogenesis occurs throughout adolescence
and adulthood.
– Sperm are produced continuously without the
prolonged interruptions in oogenesis.
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14. Ovarian Cycle
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17. Fertilization
 After ejaculation into the vagina, sperm swim to meet an egg
 Sperm live 5-7 days (need cervical mucus)
 Eggs live about 12-24 hours, so conception only occurs during this short window
 Fertilization occurs in the fallopian tube
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Events leading to
fertilization:
•Sperm binds to receptors
on zona pellucida
•Acrosomal reaction –
enzymes digest a slit
•Sperm passes through
zona
•Fusion of a single sperm’s
plasma membrane with
oocyte’s plasma membrane
•Cortical reaction: sperm
receptors destroyed in zona
so no more enter; sperm
nucleus engulfed by egg’s
cytoplasm
Fertilization occurs at
the moment the
chromosomes from the
male and female
gametes unite

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20. PREVENTION OF POLYSPERMY
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21. Initial days
• Cleavage (cell division)
• Blastocyst stage by day 4: now in uterus
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22.  Conception, fertilization of an egg by a sperm, occurs in
the oviduct.
 The resulting zygote begins to divide by mitosis in a
process called cleavage.
 Division of cells gives rise to a blastocyst, a ball of cells
with a central cavity.
 After blastocyst formation, the embryo implants into the
endometrium
 The embryo releases human chorionic gonadotropin
(hCG), which prevents menstruation
 Pregnancy, or gestation, is the condition of carrying one
or more embryos in the uterus
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23. CLEAVAGE
• Cleavage produces a ball of cells from the zygote
– Cleavage
•Rapid succession of cell divisions that produces a
ball of cells – a multicellular embryo – from the
zygote
•PLANES OF CLEAVAGE:
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24. TYPES OF CLEAVAGES
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26. GASTRULATION
• The process in which a gastrula develops from a blastula by the
inward migration of cells
• Gastrulation produces a three-layered embryo
• Gastrulation Second major phase of embryonic development;
adds more cells to the embryo; sorts the cells into distinct cell
layers.
• "It is not birth, marriage, or death, but gastrulation, which is truly
the most important time in your life." Lewis Wolpert (1986)
• Gastrula
o Three-layered stage of the embryo
o Three layers are labeled
1. Ectoderm
2. Endoderm
3. Mesoderm
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28. • Although the details of gastrulation differ between various groups of
animals, the cellular mechanisms involved in gastrulation are common to all
animals. Gastrulation involves changes in cell motility, cell shape, and cell
adhesion.
• The major types of cell movements that occur during gastrulation are:
Invagination: a sheet of cells (called an epithelial sheet) bends inward.
Ingression: individual cells leave an epithelial sheet and become freely
migrating mesenchyme cells.
Involution: an epithelial sheet rolls inward to form an underlying layer.
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29. Epiboly: a sheet of cells spreads by thinning. The movement of epithelial
sheets (usually of ectodermal cells) that spread as a unit, rather than
individually, to enclose the deeper layers of the embryo.
Intercalation: rows of cells move between one another, creating an array
of cells that is longer (in one or more dimensions) but thinner.
Convergent Extension: rows of cells intercalate, but the intercalation is
highly directional.
Delamination. The splitting of one cellular sheet into two more or less
parallel sheets
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31. Gastrula Layers Develop Into..
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32. Implantation
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Blastocyst floats for
2 days: “hatches”
by digesting zona
enough to squeeze
out
6-9 days post
conception -
burrows into
endometrium

33. Embryo Development
The embryo and placenta take shape during the first month of
pregnancy
GESTATION
Pregnancy; the carrying of developing young within the
female reproductive tract
AN OVERVIEW OF DEVELOPMENT EVENTS
Blastocyst  mammalian equivalent of a blastula
Trophoblast  outer layer that allows uterine implantation
Placenta  combination of trophoblast and maternal cells
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34. Overall: 1st Week of Development
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46. 2nd Week of Development
Development of Trophoblast
Development of Bilaminar Embryonic disc
Development of Amnion
Development of Yolk sac
Development of Sinusoids
Development of Extra embryonic mesoderm
Development of Chorions
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48. 3rd Week of Development
Gastrulation
Primitive Stalk formation, Primitive Node formation
Trilaminar Disk formation
Notochordal process, Notochord, Archentron
Oropharangeal & Cloacal membrane
Neurulation
Development of Chorionic Villi
Development of Placenta
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49. Gastrulation in birds and mammals
During gastrulation in birds and mammals, epiblast cells converge at the
midline and ingress at the primitive streak. Ingression of these cells results in
formation of the mesoderm and replacement of some of the hypoblast cells to
produce the definitive endoderm. 49

50. Development of Placenta
Placenta is an organ that connects the
developing fetus to the uterine wall to
allow –
• Nutrient uptake
• Waste elimination
• Gas exchange via the mother’s blood
supply
• Hormone production
• Formation of a barrier.
Function as a fetomaternal organ with 2
components-
1. Fetal placenta
2. Maternal placenta
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51. The “Placental Barrier”
• Sugars, fats and oxygen diffuse from mother’s blood to fetus
• Urea and CO2 diffuse from fetus to mother
• Maternal antibodies actively transported across placenta
• Some resistance to disease (passive immunity)
• Most bacteria are blocked
• Many viruses can pass including rubella, chickenpox, mono, sometimes HIV
• Many drugs and toxins pass including alcohol, heroin, mercury
• Placental secretion of hormones
• Progesterone and HCG (human chorionic gonadotropin, the hormone tested
for pregnancy): maintain the uterus
• Estrogens and CRH (corticotropin releasing hormone): promote labor
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52. Extra embryonic membranes
• Amnion  protective fluid
• Yolk sac  no yolk in humans, but same name;
nourishment for non- placentals.
• Chorion  become embryos part of placenta
 Releases human chorionic gonadotropin (hcg)
 Causes corpus luteum to stay; prevents menstruation
• Allantois  functions in waste disposal; will
become the umbilical cord
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